Field of the Invention
This invention relates generally to the field of optical transmission systems, and more particularly to optical transceivers used in optical transmission systems that use analog and digital signal process (DSP) and time synchronization techniques to predict, detect, and localize faults or other impairments to an optical fiber transmission link resulting from damage or tampering or a transmission link that is configured as a distributed sensor.
Related Art
In optical fiber transmission systems it is important to be able to predict, detect, and locate faults resulting from link damage or tampering along a transmission path. There exists a variety of methods to accomplish these tasks. For example, if a back-hoe accidentally cuts a transmission fiber conduit and severs a pair of transmission fibers, the receivers and amplifier nodes typically would have ‘loss-of-light’ indicators to notify the fault. Once a fault is detected often a crew is dispatched with an Optical Time Domain Reflectometer (OTDR) to locate the fiber cut and a repair crew is dispatched to fix the break. While the fault itself can be detected quickly, its location cannot be easily identified precisely (e.g., not better than a typical amplifier-to-amplifier distance of about 100 km) unless expensive equipment such as an OTDR is dedicated to each link. Furthermore, if there are multiple transmission fiber pairs between each link then multiple OTDRs may be required at additional real-estate, power, maintenance and cost.
Fiber cuts result in abrupt traffic outage and is therefore immediately noticeable and can be located within an amplifier but distance with simple techniques using network management protocols that are usually transmitted over a dedicated communication link that is entirely different (out of fiber) and operated at a different wavelength than the wavelength of the signal channels. Slowly occurring degradation in transmission quality resulting from impairments that are not easily noticeable such as, an event of malicious tampering of an otherwise secured transmission link by a third party for eaves dropping, are more difficult to detect. To improve security of installed transmission links, accurate and fast methods are required for detecting and precisely locating impairments and events of tampering as soon as they occur.
FIG. 1 shows an example of a prior art optical fiber transmission link 100. A typical optical fiber link comprises at least two terminals 101 and 102, connected by fiber links 103 (only one labeled for clarity) and repeater or amplifier huts 104 (only one labeled for clarity) placed at predetermined distances throughout the link. The terminals include electrical and optical transmission and switching equipment such as transponders, optical and electrical multiplexers, routers, memory and processors or computers, timing equipment, optical amplifiers, etc. Also shown below the terminal 101 is a simplified example of a DWDM (Dense Wave Division Multiplexed) transponder. Such a transponder may include a short reach interface 112 that goes to customer equipment such as a router or a switch and a DWDM interface 114 that transmits a specific wavelength(s) signals across the optical fiber link.
The optical fiber link may be placed in a network including, but not limited to a local area, a metro, a regional, or a long haul network having one or more network nodes, each node further having multiple terminals and transponders. The terminals also have other signal processing equipment 113 that perform pre-processing and post processing functions such as, framing, forward error correction (FEC), multiplexing/de-multiplexing in optical and/or electrical domain, Polarization Mode Dispersion (PMD) compensation, and other functions that are well known in the art. Not shown in FIG. 1 is that there may be 80 or more wavelengths that are optically multiplexed from multiple transponders to be transmitted on one fiber.
Also shown in FIG. 1 are different types of amplifier but designs for 2-fiber unidirectional system 114a-114c and 1-fiber bidirectional system 124a-124c, as is known in the art. One fiber systems have some advantages in that the propagation delay between the two terminals is identical and 2 fiber systems have advantages in that more information can be transmitted. The systems 114a and 124a show an amplifier but having a traditional two-stage EDFA (Erbium Doped Fiber amplifiers) 115 and 125, respectively, with mid-stage access for dispersion compensating fiber (DCF) 116 and other devices (not shown in this view). The middle row shows an alternate amplifier but design 114b and 124b with only amplifiers (115 and 125, respectively) that do not need mid-span DCF such as those used in 100 G DP-QPSK coherent transponder systems. The bottom row shows another type of amplifier but design 114c and 124c having no amplifiers or a distributed Raman Pumped amplifier for long haul transmission systems.
While a back-hoe near a distant amplifier but may cut the fiber and take down all the communication channels in a very noticeable and abrupt way, a malicious determined third party might be much more sinister in its ability to individually select one fiber to tap some of the light for eaves dropping or other malicious intentions such as spoofing or jamming. Such occurrences can happen anywhere along the fiber link including at the amplifier huts. They are very difficult and expensive to detect with technology solutions currently available.
This invention provides transponder with advanced capabilities and clock synchronization methods that may be applied in installed DWDM transmission links for in-situ monitoring of fault prediction, detection, and locating faults that may occur due to link damage or due to tampering, whether inadvertent or intentional. Advantageously, the methods according to this invention do not add significantly to equipment or operational cost of the transmission system. The concepts described in this invention may also be used to create a distributed sensor application. Such sensors may be used for example in surveillance applications to detect and localize external events such as people or vehicles passing over a section of a fiber optic link.